Method for producing a piezoelectric structural unit

ABSTRACT

A method for producing a structural unit with a prestressed piezoelectric actuator includes the steps of inserting a front end of the actuator into a hollow body, form-lockingly or force-lockingly connecting the inserted front end of the actuator to the hollow body, pressing the actuator into the hollow body with a defined force establishing a prestress of the actuator, and force-lockingly or form-lockingly connecting the actuator to the hollow body in the vicinity of a rear end of the actuator, for fixing the prestress of the actuator.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of copending InternationalApplication No. PCT/DE98/02061, filed Jul. 22, 1998, which designatedthe United States.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a piezoelectric assembly including apiezoelectric actuator prestressed and inserted into an elastic hollowbody in an extension direction between a top and a bottom cover plateconnected to the hollow body. The invention also relates to a method forproducing a structural unit with a prestressed piezoelectric actuatorand to a method for producing a hollow body used to prestress apiezoelectric actuator.

Piezoelectric actuators are used, for example, in motor-vehicletechnology to control an injection valve of an internal combustionengine. German Published, Non-Prosecuted Patent Application DE 196 53555 A1 describes a piezoelectric actuator which is preloaded by a springdevice against a cover plate of a housing. The cover plate is screwedthrough the use of a screw joint to the housing, in which the springdevice and the piezoelectric actuator are situated.

German Patent DE 38 44 134 C2 has disclosed an injection valve which hasa piezoelectric actuator as a final control element. The actuator isinserted into a hollow-cylindrical spring and is preloaded against ahousing of the injection valve by the spring. The injection valve has acomplex construction and, moreover, the preloading of the actuator isdependent on manufacturing tolerances of the housing.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a piezoelectricassembly, a method for producing a structural unit with a prestressedpiezoelectric actuator and a method for producing a hollow body forprestressing the piezoelectric actuator, which overcome thehereinafore-mentioned disadvantages of the heretofore-known devices andmethods of this general type, in which the assembly has a simpleconstruction and is optimized in terms of installation space and inwhich the methods are simple and economical.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a piezoelectric assembly, comprising anelastic hollow body; a top cover plate connected to the hollow body byone of welding and flanging, and a bottom cover plate connected to thehollow body; and a piezoelectric actuator having an extension direction,the actuator inserted into the hollow body in the extension directionbetween the cover plates for prestressing the actuator.

In accordance with another feature of the invention, the hollow body hasa given length, two butting edges and at least one connecting seamconnecting the two butting edges to one another and extending entirelyover the given length.

In accordance with a further feature of the invention, the hollow bodyhas a longitudinal direction and two butting edges associated with oneanother and disposed in the longitudinal direction, the butting edgesnot being connected to one another.

In accordance with an added feature of the invention, the hollow bodyhas apertures at least partially determining an elasticity of the hollowbody.

In accordance with an additional feature of the invention, the hollowbody is made of at least one plate formed into the hollow body and thenfixed by at least one connecting seam.

With the objects of the invention in view, there is also provided amethod for producing a piezoelectric structural unit, which comprisesinserting a front end of a piezoelectric actuator into a hollow body;force-lockingly or form-lockingly connecting the inserted front end ofthe actuator to the hollow body; pressing the actuator into the hollowbody with a defined force establishing a prestress of the actuator; andforce-lockingly or form-lockingly connecting the actuator to the hollowbody in the vicinity of a rear end of the actuator, for fixing theprestress of the actuator.

In accordance with another mode of the invention, there is provided amethod which comprises force-lockingly or form-lockingly connecting anopen end of the hollow body to a first cover plate for closing the openend, before the step of inserting the actuator; placing a second topplate on the inserted actuator and pressing the second top plate intothe hollow body with a given force; and force-lockingly orform-lockingly connecting the second cover plate to the hollow bodywhile maintaining the given force, for permanently prestressing theactuator with the hollow body.

In accordance with a further mode of the invention, there is provided amethod which comprises elastically elongating the hollow body during theprestressing of the actuator with the given force.

With the objects of the invention in view, there is additionallyprovided a method for the production of a hollow body, which comprisesproducing a plate with a defined area; forming apertures in the platefor affecting an elasticity of the plate; forming the plate into ahollow body to be used to prestress a piezoelectric actuator; and fixinga shape of the formed plate.

In accordance with a concomitant mode of the invention, there isprovided a method which comprises at least partially compacting an edgebounding at least one of the apertures.

A significant advantage of the invention is that the piezoelectricactuator is surrounded by a hollow body, and the hollow body forms asingle structural unit with the prestressed piezoelectric actuator. Theprestress of the actuator is permanently fixed during the production ofthe structural unit. The actuator thus does not need to be prestressedagain every time it is removed and installed. The structural unit istherefore easy to install, requires little maintenance and is economicalin terms of cost.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a piezoelectric assembly, a method for producing a structural unitwith a prestressed piezoelectric actuator and a method for producing ahollow body for prestressing the piezoelectric actuator, it isnevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic, sectional view of a first embodiment of aprestressed piezoelectric actuator;

FIG. 2 is a sectional view of a second embodiment of the piezoelectricactuator;

FIG. 3 is an elevational view of a sleeve with transverse slots;

FIG. 4 is an elevational view of a sleeve with oblique slots;

FIG. 5 is an elevational view of a sleeve with one oblique aperture;

FIG. 6 is an elevational view of a sleeve with a plurality of obliqueapertures;

FIG. 7 is an elevational view of a sleeve with round apertures;

FIG. 8 is an elevational view of a sleeve with an oblique connectingseam;

FIG. 9 is an elevational view of a sleeve with a wavy connecting seam;

FIG. 10 is an elevational view of a sleeve with a straight connectingseam;

FIG. 11 is an elevational view of a sleeve with a crisscross connectingseam;

FIG. 12 is a partly sectional view of an assembly apparatus forproducing a prestressed actuator; and

FIG. 13 is a view similar to FIG. 12 of another assembly apparatus forprestressing an actuator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawings in detail and first,particularly, to FIG. 1 thereof, there is seen a piezoelectric actuator1 which includes a plurality of individual piezoelectric elementsstacked one upon the other. The piezoelectric actuator 1 is introducedinto a hollow body, mould, shape or form 2. The hollow body or mould 2is, for example, a sleeve with an annular cross section or arectangular-section tube profile. The hollow body or mould 2 ispreferably matched in shape to the shape of the piezoelectric actuator1. A bottom end of the piezoelectric actuator 1 rests on a bottom coverplate 4, which is force-lockingly and/or form-lockingly connected to thehollow body or mould 2. In this illustrative embodiment, the bottomcover plate 4 is form-lockingly connected to the hollow body or mould 2by a weld 5. A form-locking connection is one which connects twoelements together due to the shape of the elements themselves, asopposed to a force-locking connection, which locks the elements togetherby force external to the elements.

A top end of the piezoelectric actuator 1 rests against an upper coverplate 3, which is likewise form-lockingly and/or force-lockinglyconnected to the hollow body or mould 2. In this illustrativeembodiment, the top cover plate 3 is form-lockingly connected to thehollow body or mould 2 by a weld 5.

The top cover plate 3 has passages 7 through which contact pins 6 of thepiezoelectric actuator are passed. The contact pins 6 are used to makethe piezoelectric actuator capable of being activated for extension orexpansion. The piezoelectric actuator 1 is prestressed with a definedforce by the hollow body or mould 2 and the bottom and top cover plates3, 4 so as to resist deflection. In order to achieve this, the hollowbody or mould 2 is manufactured from a correspondingly elastic material.Preferred materials are those with a low modulus of elasticity, such asa copper-beryllium alloy (CuBe₂) and/or those with a high strengthcharacteristic, such as spring steel.

A further improvement in the elastic properties of the hollow body ormould 2 is achieved by making apertures in the hollow body or mould 2.Different shapes of the apertures are illustrated in greater detail inFIGS. 3 to 7.

FIG. 2 shows a prestressed piezoelectric actuator 1 which, as in FIG. 1,is clamped between a top cover plate and a bottom cover plate 3, 4 by ahollow body or mould 2. In this example, the connection between thehollow body or mould 2 and the top and bottom cover plates 3, 4 iseffected through the use of flanges 8. For this purpose, the top andbottom cover plates 3, 4 preferably have recesses 26 into whichrespective flanged top and bottom edge regions of the hollow body ormould 2 engage. In a simple embodiment, no recesses 26 are provided andthe top and bottom edge regions are simply flanged around the top andbottom edges of the top and bottom cover plates 3, 4.

A significant advantage of the structural units illustrated in FIG. 1and FIG. 2, which are constructed of the top and bottom cover plates 3,4, the hollow body or mould 2 and the piezoelectric actuator 1, is thatthe prestress of the piezoelectric actuator is permanently fixed throughthe use of the fixed connection to the hollow body or mould 2. Thismeans that there is no need for readjustment during the entire servicelife of the actuator.

The piezoelectric actuator 1, the hollow body or mould 2 and the top andbottom cover plates 3, 4 form a compact structural unit which can betransported without problems and can be installed in a correspondingfinal control element, in particular an injection valve, in a simpleoperation. The structural unit 1, 2, 3, 4 can be removed from the finalcontrol element without any change in the prestress of the piezoelectricactuator 1. In addition, the structural unit can be manufactured atrelatively reasonable cost.

FIG. 3 shows a hollow body or mould 2 that forms a cylindrical sleeve inwhich transverse slots 10 are made perpendicularly to the longitudinalaxis. The number and length of the transverse slots 10 are such that theelasticity of the hollow body or mould 2 is matched to the desiredmanner of operation of the prestressed piezoelectric actuator accordingto FIGS. 1 and 2.

FIG. 4 shows a hollow body or mould 2 that forms a cylindrical sleeve inwhich oblique slots 11 are made obliquely to the longitudinal axis ofthe hollow body or mould 2.

FIG. 5 shows a hollow body or mould 2 that forms a cylindrical sleeve inwhich an oblique aperture 12 running in a spiral around the hollow bodyor mould 2 is made. In this case, at least part of the hollow body ormould 2 is in the form of a cylindrical spiral.

FIG. 6 shows a hollow body or mould 2 that forms a cylindrical sleeve inwhich a plurality of slots 13 disposed parallel to one another and in aspiral relative to the longitudinal axis of the hollow body or mould 2are made. Due to the slots 13, at least part of the hollow body or mould2 forms a plurality of cylindrical spring elements in the form ofspirals.

FIG. 7 shows a hollow body or mould 2 that is constructed as acylindrical sleeve in which circular apertures 14 are made.

The number and geometry of the apertures and slots in FIGS. 3 to 7 arechosen in such a way that the elasticity of the hollow body or mould 2in the longitudinal direction, i.e. parallel to the direction ofdeflection of the piezoelectric actuator 1, is matched to the desiredmanner of operation of the piezoelectric actuator 1.

The hollow body or moulds 2 illustrated in FIGS. 3 to 7 are preferablyconstructed as thin-walled hollow body or moulds which are produced froma simple, thin-walled and preferably drawn tube.

An advantageous method for producing the hollow body 2 includes makingapertures and/or slots in a plate of defined thickness and defineddimensions in a first operation. The shape of the plate offers theadvantage of making the plate readily accessible for the introduction ofthe apertures, and the apertures can be made in a wide variety ofgeometries, numbers and configurations, for example by inexpensivepunching.

An edge region of an aperture is preferably slightly plasticallydeformed, resulting in hardening of the plate due to an introducedinternal compressive stress. A corresponding hardening can also beachieved, for example, by shot peening the edge regions of the aperture.

This has a highly strength-enhancing effect, particularly in the case ofdynamic loading of the hollow body or mould 2. This is achieved, forexample, if the punch with which the apertures are made in the plate iswidened to a larger cross section, after the punching cross section, asseen in the direction of punching. As a result, the desired aperture ispunched out of the plate first and then the edge region of the apertureis compacted. This introduces an internal compressive stress into theedge region of the aperture which results in good fatigue strength.During this process, the thickness of the edge is preferably increased.The edge may be bent slightly out of the plane of the plate, dependingon the type of working. This can, for example, be an indication ofcompaction of the edge.

The plate is then formed to provide the desired hollow body or mould 2and fixed in the hollow body or mould 2 through the use of a weld. Aseam which is formed in this process is preferably welded with a laser.Non-uniformities in the weld are eliminated by a heat treatment.

As an alternative, it is also possible for the tubular shape to be fixedby connecting the hollow body or mould 2 to the top and bottom coverplates 3, 4. In this case, butting edges, which are usually welded, arenot connected to one another. The butting edges illustrated in FIGS. 8to 11, which form seams 15, 17, 16, 18, are thus not connected to oneanother in this illustrative embodiment. An advantageous distribution ofthe spring and pressure forces is thereby achieved.

FIGS. 8 to 11 show various forms of seams 15, 16, 17, 18 which connecttwo butting edges of a formed plate and thus fix the hollow body ormould 2. FIG. 8 shows an oblique seam 15 disposed obliquely to thelongitudinal axis of the sleeve shape 2. FIG. 9 shows a wavy seam 17,which is disposed symmetrically with respect to the longitudinaldirection of the hollow body or mould 2 and is disposed in the form of asine wave parallel to the longitudinal direction of the sleeve shape 2.FIG. 10 shows a longitudinal seam 16 disposed parallel to thelongitudinal direction of the hollow body or mould 2. FIG. 11 shows acrisscross seam 18 disposed symmetrically with respect to thelongitudinal axis of the hollow body or mould 2. The crisscross seam 18is disposed in the form of a zigzag line in the longitudinal directionrelative to the hollow body or mould 2. The seams 15, 16, 17, 18 arepreferably welded.

The spring rate of the hollow body or mould 2 can be adjusted withinwide limits through the use of the following parameters: inside andoutside diameter of the sleeve mould, wall thickness, material withsuitable modulus of elasticity, number of apertures, geometry of theapertures (holes, grooves, spiral grooves, etc.), and configuration ofthe apertures (horizontally, vertically, obliquely and, in the case ofspiral grooves: slope, number of flights, groove width, etc.).

The apertures are, for example, made by drilling, punching, milling,erosion or electrochemical methods.

FIG. 12 shows an assembly configuration through the use of which apiezoelectric actuator 1 is installed in a sleeve-shaped hollow body ormould 2 with a precisely defined force. For this purpose, one end of thehollow body or mould 2 is form-lockingly and/or force-lockinglyconnected to a top cover plate 3 having passages 7. The hollow body ormould 2 has a bearing edge 24 which points radially outward at anotherend. The hollow body or mould 2 is inserted, top cover plate 3 first,into an assembly fixture 20. The assembly fixture 20 has an assemblyrecess 21 which matches the cross section of the hollow body or mould 2in such a way that the hollow body or mould 2 can be inserted into theassembly recess 21 with the bearing edge 24 supported in a bearingregion 27 of the assembly fixture 20. A piezoelectric actuator 1 is theninserted, contact pins 6 at a front end first, into the hollow body ormould 2. A bottom cover plate 4 is then inserted into the hollow body ormould 2 and pressed into the hollow body or mould 2 with a defined forceby an assembly device 23 using a punch 22, so that the hollow body ormould 2 is thereby elongated elastically.

The defined force corresponds to the desired prestress of thepiezoelectric actuator 1. While maintaining the force, the bottom coverplate 4 is then force-lockingly and/or form-lockingly connected to thehollow body or mould 2, preferably by being welded to it. This connectsthe actuator to the hollow body in the vicinity of the rear end of theactuator and fixes the prestress of the piezoelectric actuator 1.

FIG. 13 shows a second variant of the assembly configuration. Incontrast to the configuration in FIG. 12, the hollow body or mould 2remains unloaded during assembly, and the bent-over bearing edge 27shown in FIG. 12 can be omitted. For this purpose, one end of the hollowbody or mould is form-lockingly and/or force-lockingly connected to thetop cover plate 3 having the passages 7, the piezoelectric actuator 1 isintroduced into the hollow body or mould 2 and, as shown in FIG. 13,inserted with the contact pins of the piezoelectric actuator 1 enteringinto depressions that are provided for this purpose in a baseplate 19.The bottom cover plate 4 is then placed on the actuator 1, pressed intothe hollow body or mould through the use of a punch 22 of an assemblyfixture 20, and then form-lockingly and/or force-lockingly connected tothis hollow body or mould, preferably by being welded to it. The forcewhich is used for pressing in compresses only the piezoelectric actuator1 and the cover plates 3, 4. After the relief of the load on theassembly fixture 20, the actuator 1 expands and, in the process,stresses the hollow body or mould 2. The force used for pressing in mustbe calculated, while taking the stiffnesses of the actuator 1 and thehollow body or mould 2 into account, in such a way that a definedprestressing force is established after the relief of the load on thesystem.

Length tolerances of the piezoelectric actuator 1 which are inherent inproduction can preferably be compensated for by grinding the bottomcover plate 4. Grinding can furthermore be employed to correct anydeviation from parallelism of the end surfaces of the piezoelectricactuator 1. For this purpose, the bottom cover plate 4 should be groundat a corresponding angle.

We claim:
 1. A method for producing a piezoelectric structural unit,which comprises: a) inserting a front end of a piezoelectric actuatorinto a hollow body having a longitudinal direction; b) force-lockinglyor form-lockingly connecting the inserted front end of the actuator tothe hollow body; c) pressing the actuator into the hollow body with adefined force in the longitudinal direction for prestressing andestablishing a desired prestress of the actuator in the longitudinaldirection; and then d) force-lockingly or form-lockingly connecting theactuator to the hollow body in the vicinity of a rear end of theactuator, for fixing the desired prestress of the actuator.
 2. Themethod according to claim 1, which further comprises: force-lockingly orform-lockingly connecting an open end of the hollow body to a firstcover plate for closing the open end, before the step of inserting theactuator; placing a second top plate on the inserted actuator andpressing the second top plate into the hollow body with a given force;and force-lockingly or form-lockingly connecting the second cover plateto the hollow body while maintaining the given force, for permanentlyprestressing the actuator with the hollow body in the longitudinaldirection.
 3. The method according to claim 1, which compriseselastically elongating the hollow body in the longitudinal directionduring the prestressing of the actuator.